Extreme droughts triggered the great barbarian invasion of Roman Britain in the mid-4th century AD

A study led by the University of Cambridge has revealed that a series of extreme droughts between the years 364 and 366 AD may have been a determining factor in the so-called Great Barbarian Conspiracy of the year 367, one of the most severe attacks on Roman rule in Britain since Boudica’s rebellion three centuries earlier.

The analysis, based on the growth rings of oaks from southern Britain, made it possible to reconstruct temperature and precipitation levels at the time and connect them to Roman accounts of food shortages and military collapse.

During the conspiracy, the Picts attacked by land and sea following a rebellion within the Roman ranks at Hadrian’s Wall. At the same time, the Scotti (from present-day Ireland) and the Saxons (from the European continent) invaded from the west and south, respectively.

Climatological and historical background for the Barbarian Conspiracy. (a) Vaticanus Latinus 1873, one of two surviving independent manuscripts of Ammianus’ Res Gestae in which the events of the 367 CE Barbarian Conspiracy are described, re-discovered in the Abbey of Fulda, Germany in 1417 CE (b) Hadrian’s Wall, one of the sites where major incursions likely occurred after the garrison rebelled and invaders were allowed to pass through. (c) The broad movements of tribes outlined by Ammianus in the winter of 367 CE. (d) Early Common Era hydroclimatic records covering 350–500 CE over varying spatial scales. Credit: C. Norman et al.

Several Roman commanders were captured or killed, and some soldiers deserted and joined the invaders. The resulting anarchy took two years to control and marked the beginning of the end of Roman administration in Britain, which would withdraw permanently around the year 410.

According to the study, published in Climatic Change, average rainfall between April and July, the key growing season, was 51 mm in southern Britain. However, during the critical years it dropped drastically: 29 mm in 364, 28 mm in 365, and 37 mm in 366. This sequence of exceptionally dry summers was devastating for agriculture, particularly for crops like emmer wheat and six-row barley, which were essential for the province’s food supply.

Professor Ulf Büntgen explained that the grain shortage weakened the social contract between the Roman army and its soldiers, which may have triggered rebellions and desertions. The lack of grain would also have compromised supply to Hadrian’s Wall, thereby facilitating the entry of the Picts into northern Britain. Britain’s geographical isolation and the diversion of military resources to the Rhine frontier limited the Empire’s ability to respond.

Pictish warrior in an illustration from 1857. Credit: Public domain / Wikimedia Commons

The researchers found that this chain of droughts was unique within the period 350–500 AD, and that no other region in northwestern Europe experienced similar conditions. This undermines the idea that the invading peoples moved due to hunger in their own lands and strengthens the hypothesis that they took advantage of Britain’s internal weakness.

Moreover, by expanding the climate-conflict analysis to the rest of the Roman Empire between 350 and 476 AD, the scientists detected a significant correlation between dry years and battles. Tatiana Bebchuk noted that this relationship between extreme climate, hunger, and social conflict also has relevance in today’s world.

Theodosius was the one sent to suppress the Barbarica conspiratio, as we recounted in our article The Great Conspiracy, the coordinated attack of several barbarian peoples on the Romans of Britain and northern Gaul: Before the end of the year, the barbarians had been expelled and the soldiers who did not return to the ranks were captured and executed. Their leader was Valentinus, a Pannonian brother-in-law of the praetorian prefect Maximinus, who had been assigned to Britain precisely to remove him from Rome due to an uncertain offense. There is no evidence that Valentinus was part of the Great Conspiracy, but he did take advantage of the anarchic situation to try to rally all the discontented and oppose Theodosius, which led him to the gallows; albeit discreetly, to avoid possible riots.